The present invention relates to a process and an apparatus for subjecting fluids to irradiation by high intensity ultrasound.
If a liquid is subjected to a high ultrasonic intensity, that is to say an intensity greater than about 0.3 W/cm2 at a frequency typically in the range 10 kHz to 100 kHz, then there is a significant deposition of energy into the liquid through attenuation and non-linear effects. This may lead to physical changes (for example streaming, mixing, or emulsification) or to chemical changes, and the present invention is principally concerned with the latter, which may be referred to as sonochemistry. The most significant sonochemical effects are usually associated with cavitation in the liquid, especially transient vaporous cavitation, which typically only occurs at an ultrasonic intensity above a threshold which is typically above 0.3 W/cm2 and is different for different liquids.
An apparatus for subjecting a liquid to a very high intensity is described in GB 2 243 092 B (UKAEA), in which a transducer is attached to one end of a tapered resonant coupler, the coupler having a nodal flange; this flange is attached to a sleeve projecting from the wall of a pipe containing the liquid, and the spade around the sides and end of the coupler within the sleeve is filled with a coupling liquid such as olive oil. The use of such a nodal flange mounting enables a transducer of higher power to be used, but the apparatus is somewhat complex. It has also been suggested, for example in U.S. Pat. No. 4,433,916 (Hall), to attach a plurality of acoustic transducers directly to the wall of a cylindrical vessel containing a fluid, with a specified phase difference between the acoustic signals applied to adjacent transducers. Similarly, U.S. Pat. No. 4,369,100 (Sawyer) describes attaching ultrasonic transducers to the outer wall of an annular duct so that both the inner and outer walls are set in resonance. And U.S. Pat. No. 2,578,505 (Carlin) also describes a pipe with several transducers attached to the surface arranged in one or more circumferential rings.